This application claims priority from EP01301864.3 filed Mar. 1, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to lithographic apparatus and more particularly to lithographic apparatus using reflective masks.
2. Background of the Related Art
In general, a lithographic extreme ultraviolet radiation lithographic apparatus include, a reflective mask, a gripper to hold the mask, a radiation system for providing the projection beam of extreme ultraviolet electromagnetic radiation, a mask table for holding the reflective mask, the mask serving to pattern the projection beam according to a desired pattern upon reflection of the projection beam at the mask so as to yield a patterned projection beam, a substrate table for holding a substrate and a projection system for projecting the patterned projection beam onto a target portion of the substrate.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Conventionally, the mask table has been positioned such that radiation is passed from the illumination system through the mask, the projection system and onto the substrate. Such masks are known as transmissive masks since they selectively allow the radiation from the illumination system to pass through, thereby forming a pattern on the substrate. Such masks must be supported so as to allow the transmission of light therethrough. This has conventionally been achieved by using a vacuum in the table at a perimeter zone of the mask so that the atmospheric air pressure clamps the mask to the table.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light of 365 nm, 248 nm and 193 nm generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) radiation and possible sources include laser-produced plasma sources, discharge sources or synchrotron radiation sources, examples of which are, for instance, disclosed in European patent applications EP 1 109 427 A and EP 1 170 982 A, incorporated herein by reference.
Since no materials are known to date to be sufficiently transparent to EUV radiation, a lithographic projection apparatus employing EUV radiation is envisaged to employ a reflective mask having a multilayer coating of alternating layers of different materials, for instance, in the order of 50 periods of alternating layers of molybdenum and silicon or other materials, such as, for instance, disclosed in European patent application EP 1 065 532 A, incorporated herein by reference. The size of the features to be imaged in EUV lithography makes the imaging process very sensitive to any contamination present on the mask. It is foreseen that any contaminant particles having a dimension in the order of 50 nm will result in defects present in devices fabricated in the substrate. Conventionally, the patterned side of the reticle is covered by a so-called pellicle. Any contamination will then accumulate on the pellicle surface at some distance from the mask pattern and will therefore not be (sharply) imaged onto the substrate, making such masks having pellicles less sensitive to contamination. Pellicles cannot be employed for EUV radiation since they will not be sufficiently transparent to EUV radiation. Particle contamination on the pattern-bearing reflective surface of the mask would therefore lead to defective devices fabricated and must be prevented.
Further, the reflective mask is envisaged to be held at its backside on the mask table by electrostatic forces on a mask-bearing surface to be able to meet the very stringent requirements for EUV mask positioning. Any contaminant particles present in between the backside of the mask and the mask-bearing surface of the mask table will result in irregularities of the reflective mask surface. Since the projection system will be non-telecentric on the object side because a reflective mask is used (more information on this problem can be derived from European patent application EP 1 139 176 A, incorporated herein by reference), any irregularity in the surface figure of the reflective mask surface will translate into a local shift of the pattern imaged onto the substrate. As a result, the imaged layer may not line up with earlier layers that have been processed in the substrate, again leading to defective devices fabricated. Therefore, particle contamination on the backside surface of the mask must be prevented.
Handling of the mask is required to bring it into and out of various types of equipment, such as multilayer deposition equipment, mask pattern write equipment, mask inspection equipment, lithographic projection apparatus, but also a mask storage box to be able to transport the mask between these various types of equipment. Further, inside the various types of equipment, handling of the mask is required, for instance, to take the mask to a mask table for processing, to store it in a mask library and to take the mask out again. Conventionally, a mask is held in handling procedures by a mechanical or vacuum-operated clamp. Such conventional methods appear to constitute a large source of particle generation each time a mask is taken over by another gripper of some handling device. It has been found that gripping will cause particles of various sizes to become released from the gripping area. The inventors have determined that microslip of the contacting surfaces seems a major cause for such particle generation. The inventors have further determined that a more delicate handling method is therefore required for EUV masks and that any mechanical contact between mask and a gripper should be designed such that particle generation during handling and take-over of the mask (reticle) is largely minimized or prevented.
Proper alignment of the mask is often required in the various types of above equipment. Conventionally, this is achieved by clamping the mask, determining alignment, releasing the clamp, adjusting the position of the mask using another gripper, clamping the mask again with the first clamp, determining alignment again, and keep repeating this sequence until satisfactory alignment has been achieved. Particles will be generated each time the mask is clamped and unclamped and an improved method of handling the mask is required to alleviate this problem.